Perhaps your local school, YMCA or Boys and Girls club could use them. Our YMCA sometimes puts out a container asking for donations of equipment like hockey sticks and tennis racquets from time to time.
I saw an article a while ago in either TENNIS or SMASH magazine (i think it was SMASH) about a kid who started a racquet donation organization. I tried looking for it but couldn't find it.
I think he gives them to inner-city kids or kids in third world countries.